kbuild: show warning if 'make headers_check' is used

Since commit 7ecaf069

 ("kbuild: move headers_check rule to
usr/include/Makefile"), 'make headers_check' is no-op.

This stub target is remaining here in case some scripts still invoke it.
In order to prompt people to remove stale code, show a noisy warning
message if used. The stub will be really removed after the Linux 5.15
release.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>